"it is a remarkable circumstance that every one of the twenty-eight tuff-craters which were examined, had their southern sides either much lower than the other sides, or quite broken down and removed. As all these craters apparently have been formed when standing in the sea, and as the waves from the trade wind and the swell from the open Pacific here unite their forces on the southern coasts of all the islands, this singular uniformity in the broken state of the craters, composed of the soft and yielding tuff, is easily explained"
ö Darwin, Voyage of the Beagle, Chap. 17
Galápagos Climate:
- Relatively mild (19-30° C [66-86° F] air temperatures, 22-25° C [72-77° F] sea temperatures)
- Very dry (annual rainfall of less than 200 mm [7.9 inches])
- Two primary seasons in Galápagos:
- January-early June: warm/wet season
- Warmer temperatures
- Winds from the east
- Gentle seas
- Clear skies, with occasional heavy rains
- Late June-Decenter: garúa or cool/dry season
- Cooler temperatures
- Winds from the southeast
- Choppier seas
- Garúa (mist) forms due to inversion layer 300-600 m above sea level
- January-early June: warm/wet season
- Highlands get more water from rain or garúa than lowlands
- Rain concentrates on southern and eastern slopes of highlands
Climatology
As everywhere, weather is controlled primarily by oceanography, and shallow
oceanography (upper 1 km) is controlled by wind. So let's start with global air
circulation.
Lower 10-18 km of atmosphere is the troposphere. It is warmest at the base (where warmed by land surface) and cools further up (due to distance from heat source and to expansion (decreased pressure)).
As land surface heats up due to sunlight, warm shallow air rises. Since nature abhors a vacuum, other air must come in replace it.
Since Earth's surface is curved (sphere), get latitudinal effects:
- Equator has more direct sunlight
(more photons/unit area) and less atmosphere to penetrate than poles (less photons/unit area)
- Consequently, equator is warmed more than poles
- Equator tends to gain energy (more incoming sunlight than outgoing heat), poles lose it, 40° latitude breaks even
- So, all other things being equal, air rises at the equator, sinks at the poles, and there would be one big circulation cell
However, Earth also rotates, so it generates Coriolis effects:
- Object (including air mass)moving from poles toward equator will be deflected from straight line target, due to rotating Earth
- Similarly, object from equator moving poleward would move off of intended straight line target
- Net result: deflection pattern which is clockwise in northern hemisphere, counter clockwise in southern hemisphere
In cases of air masses, combined effect of altitudinal, latitudinal, and Coriolis effects generates Hadley circulation cells.
Global circulation pattern of Hadley cells in turn drives global major wind patterns. Equator represents the convergence of the Trade Winds.
OceanographyWind patterns drive shallow (<1 km) oceanic circulation. Because of Coriolis forces, net motion of ocean water is perpendicular to wind direction (90° to right in northern hemisphere, 90° to the left in the southern hemisphere). This sets up large gyres (huge circulation patterns) as well as smaller countercurrents in the seas. The combination of all these produce Global circulation patterns.
Galápagos Archipelago lies at confluence of several major currents:
- Peru Current and Humbolt (Peru Coastal) Currents: eastern parts of the South Pacific Gyre, bringing cold Antarctic water towards equator
- South Equatorial Current: northern part of the South Pacific Gyre
- Panama Current (also called Niño Flow), seasonal, off of northern South America: more on this later
Major effects:
Because flow is away from a landmass, get upwelling:
- Net flow of surface water is away from land
- Deep water wells up to replace that which is blown away (Nature abhors a vacuum)
- Deep water has far more dissolved nutrients than shallow water (used up by photosynthesis)
- Provides food for a LOT of organisms
Long and short term climate fluctuations:
Quaternary glaciationIce Ages.
Not simply a prolonged period of cold, but instead a fluctuation between two major states:
- Glacial
- Accumulation of ice in continental and alpine glaciers greater than melting, so increase and expand
- Water that makes up glaciers is taken out of the hydrosphere, so sea level drops by as much as 120 m
- Interglacial
- Melting of ice in continental and alpine glaciers greater than accumulation, so decrease and shrink
- Water that had been trapped in glaciers is released by hydrosphere
- If all Greenland's glaciers melt, sea level rises 6 m; with Antarctica added, total is 30 m sea level rise
Effects of glacial-interglacial cycles on Galápagos
- Temperature effects not very important (temperatures at equator least affected by Ice Age cycles)
- Sea level drop allows connections between currently isolated islands (for example, much of the central archipelago)
- Sea level rise may separate what are currently multiple cones on the same island
- Evidence of sea level drop in the past: rounded boulders in Española show that much of it was underwater during peak interglacials
Shorter Term climate changes: El Niño Southern Oscillation (ENSO) events
Periodically, starting around Christmas time, a major disruption of coastal Ecuadorian and
Peruvian fisheries is disrupted. Because of timing shortly before Christmas, called "The Child".
Seen locally as warming of ocean water, and consequently bad fishing and stronger storms.
- Pressure differential in the South Pacific decreases (cause uncertain)
- Trade winds lessen
- Westward-trending currents at equator lose strength
- With less water being pulled away from coastline, upwelling is reduced or stops
- Warm tropical water from central and western Pacific sloshes back against South America
- With no upwelling:
- Lowered nutrient levels
- Sea temperatures rise
- Productivity drops, as plankton dies
- With warmer water:
- Local air temperature rises
- Rain storms, some severe, will form
For coastal South American communities, El Niño years are very tough: fishing gets bad, storms cause damage, even farming suffers if bird guano is a major fertilizer (birds fly off to better fisheries, so no new guano).
An intense version of the opposite system was called "Anti-El Niño", but since this would basically mean "Anti-Christ", it has been re-dubbed " La Niña" ("the Girl"). It causes greater dryness in the region.
El Niño years in Galápagos:
- Decreased upwelling leads to reduced food availability in the seas
- Some fish, sea birds, and sea mammals leave
- Other fish, sea birds, sea mammals, and marine iguanas suffer mass deaths
- Increased rainfall leads to greater productivity on land
- Plants, and land animals, flourish
La Niña years in Galápagos has opposite effect (great for sea life, tough for land)
